[PATCH RFC v2 0/3] vdpa: dedicated descriptor table group

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Following patchset introduces dedicated group for descriptor table to
reduce live migration downtime when passthrough VQ is being switched
to shadow VQ. This RFC v2 is sent to incorporate the early feedback
from reviewers on the uAPI and driver API part of changes, the
associated driver patch set consuming ths API will come around
soon along with formal submission of this series.

Some initial performance data will be gathered using the real
hardware device with mlx5_vdpa. The target goal of this series is to
reduce the SVQ switching overhead to less than 300ms on a ~100GB
guest with 2 non-mq vhost-vdpa devices. The reduction in the downtime
is thanks to avoiding the full remap in the switching.

The plan of the intended driver implementation is to use a dedicated
group (specifically, 2 in below table) to host the descriptor tables
for data vqs, different from where buffer addresses are contained (in
group 0 as below). cvq does not have to allocate dedicated group for
descriptor table, so its buffers and descriptor table would always
belong to the same group (1 in table below).

              |  data vq | ctrl vq
vq_group      |    0     |    1
vq_desc_group |    2     |    1


Si-Wei Liu (3):
  vdpa: introduce dedicated descriptor group for virtqueue
  vhost-vdpa: introduce descriptor group backend feature
  vhost-vdpa: uAPI to get dedicated descriptor group id

 drivers/vhost/vdpa.c             | 27 +++++++++++++++++++++++++++
 include/linux/vdpa.h             | 11 +++++++++++
 include/uapi/linux/vhost.h       |  8 ++++++++
 include/uapi/linux/vhost_types.h |  5 +++++
 4 files changed, 51 insertions(+)


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